Anything higher than 5V.The higher the voltage the more current the flame will conduct.Making the resistor higher will make any change in current a larger change in voltage and the easier it is to detect.

sorry to bring this back up again i had got this working but its extremely unstable as there is not much movement on the arduino, i was using 1500k resistor to even get it to work i cant seem to add any more than that or it seems to go the other way so i dont know how else we could make this work more reliably and give us more of a range on the arduinoi measured the resistance of the probe and flame was 1400k and my supply voltage i was running 17.11 and im getting back on the other end 1.65V so hell of a drop?

ok thanks for that i have had a bit of a play and come up with thishttp://s906.photobucket.com/albums/ac270/Joes-110/?action=view&current=Untitled.pngbut now we have gone to the other way now it is to sensitive and can not seem to dye down any idea?

Three things wrong with that circuit.1) It only has a gain of 2, see how to calculate the gain of a non inverting op amp here:- http://en.wikipedia.org/wiki/Operational_amplifier2) You in effect have a floating input, that is the input is very high impedance and is picking up all sorts of interference.3) That sort of circuit will only amplify voltages, you are in effect putting a variable resistance on the input you are not inputting a voltage.

Try this. +ve input of amp connected to two 1M resistors, one goes to +5 and the other ground. Then feed this point and ground at your flame.Change the resistor that goes from the -ve input to ground to a 10K. What sort of op amp are you using.

Sorry I forgot about the DC bias. Take the resistor that goes to the -ve of the op amp to ground and disconnect it from ground an put it to the wiper of a 10K pot with the two ends to ground and +ve.This will allow you to adjust the output to be mid range with no flame, then you should see it move in the flame.

No the end of the pot goes to the +ve supply not the +ve input of the op amp.And the 100K still goes to the -ve input of the op amp.And you still need the 10K resistor going between the wiper and the -ve input of the op amp.